Atypical cell forms overproducing extracellular substances in population of cycad cyanobionts

The ultrastructure of the cyanobionts of the greenhouse-grown cycads Cycas circinalis, Ceratozamia mexicana, and Encephalartos villosus was studied. The cyanobiont microcolonies grown in the intercellular space of the cyanobacterial zone of cortical parenchyma in the cycad coralloid roots contained two specific forms of vegetative cells with a reduced cell wall, namely, protoplasts and spheroplasts. The protoplasts and spheroplasts exhibited ultrastructural changes indicating the overproduction of two extracellular substances, one of which resembled the mucilage polysaccharides and the other was proteinous. The substances were likely to be synthesized intracellularly and then be excreted with the aid of surface vesicles or by channels in the cytoplasmic membrane to form, respectively, a slimy extracellular matrix and an additional electron-opaque envelope around the cell. At the late developmental stages, the excretion of these substances was accompanied by degradative changes in the cells, leading eventually to cell death. The physiological role of these specific cell forms and the factors that induce their development and death in the cell populations of cyanobionts are discussed.